Manufacture of vitreous silica bodies

ABSTRACT

A green body formed of substantially -60 mesh particles of crystalline silica and/or glass having a SiO2 content of at least 90 wt. percent is initially fired in ambient atmosphere of helium to temperature of about 1,600*-1,725* C. so as to leave about 520 percent helium-filled porosity with pore diameters of about 0.05-0.5 micron and communicating with the ambient atmosphere at the surface of the body. Then, the helium is substantially replaced with argon and firing is continued to temperature of about 1,730*-2,200* C. at a rate to concurrently effect escape of the helium from the pores substantially via the communication thereof with the ambient argon atmosphere and the virtual collapse and elimination of the pores. Resultant product is virtually free of occluded bubbles.

United States Patent [191 Nicastro, Jr. et a1.

[ Nov. 27, 1973 MANUFACTURE OF VITREOUS SILICA BODIES [75] lnventors:Carmine A. Nicastro, Jr., Big Flats;

Roelant S. L. Vander Noordaa, Corning; William A. Ward, Painted Post,all of N.Y.

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: June 28, 1971 [2]] Appl. No.: 157,710

[52] US. Cl 65/18, 65/32, 65/134, 65/DIG. 8 [51] Int. Cl C03b 23/20,C03b [58] Field of Search 65/32, 134, DIG. 8, 65/ 18 [56] ReferencesCited UNITED STATES PATENTS 2,612,726 10/1952 Nordberg 65/32 2,612,72710/1952 Nordberg 65/32 3,113,008 12/1963 Elmer 65/32 X 3,149,946 9/1964Elmer 65/32 3,320,045 5/1967 Weiss et a1 65/32 X Primary Examiner-FrankW. Miga Attorney-Clarence R. Patty, Jr. et al.

[5 7] ABSTRACT A green body formed of substantially-60 mesh particles ofcrystalline silica and/or glass having a $102 content of at least 90 wt.percent is initially tired in ambient atmosphere of helium to t emperature of about 1,600-1,725 C. so as to leave about 5-20 percenthelium-filled porosity with pore diameters of about 0.05-0.5 micron andcommunicating with the ambient atmosphere at the surface of the body.Then, the helium is substantially replaced with argon and firing iscontinued to temperature of about l,7302,200 C. at a rate toconcurrently effect escape of the helium from the pores substantiallyvia the communication thereof with the ambient argon atmosphere and thevirtual collapse and elimination of the pores. Resultant product isvirtually free of occluded bubbles.

10 Claims, N0 Drawings 1 MANUFACTURE OF VITREOUS SILICA BODIESCROSS-REFERENCE TO RELATED APPLICATION Our copending US. Pat.application Ser. No. 157,301, filed on the same data as was thisapplication and entitled PROCESS IMPROVEMENT FOR MAN- UFACTURING CLOSETOLERANCE VITREOUS SILICA ARTICLES discloses a unique mandrel supportand gravity-stretch technique that can be used in conjunction with theinvention claimed in this application for the manufacture of vitreoussilica crucibles and like bodies with consistently reproducible shapeand size while also being virtually free of occluded bubbles.

BACKGROUND OF THE INVENTION It has long been recognized that a seriousdisadvantage of vitreous silica bodies produced by firing green bodiesformed of suitable particulate materials is the presence therein ofoccluded bubbles, whose origin is the pores of the green bodies. Amongthe techniques employed heretofore to minimize such occluded bubbles isthe firing of the green bodies to fusion temperature thereof (i.e.,above the melting point of cristobalite which is variously reportedbetween about 1,723 C. and l,728 C.) in an ambient atmosphere of helium(e.g. see US. Pat. No. 2,038,627 and German Pat. No. 621,936). However,use of that technique has not .proved it capable of rendering thevitreous product virtually free of occluded bubbles as has long beendesired. The noted German patent acknowledges that a significant numberof such bubbles remain and suggests they be removed by the further stepof hot plastic working of the vitreous silica mass. On the other hand,the noted United States patent suggests that, after the green body hasbeen fired to a fused or viscous molten condition, the helium atmospherebe replaced with hydrogen or similar gas that is nonreactive with moltensilica and the furnace components, whereby diffusion of helium throughthe viscous silica mass from trapped bubbles therein is somewhataccelerated. Nevertheless, vitreous silica products made by the latterprocedure still contain undesirable numbers of occluded bubbles, whichare in noticeable. concentrations in the central portions of relativelythick bodies and are randomly scattered within relatively thin bodies.From our experience with the latter procedure, total firing periods of 1hour (and even somewhat more than 1 hour) are not sufficient to enablevirtual disappearance of the bubbles by helium diffusion through theviscous silica mass. It appears questionable that substantially longerfiring periods would make any significant further reduction in theamount of bubbles without becoming an uneconomically prohibitive processfor commercial purposes.

In these old processes, it isapparent that diffusion of helium throughthe viscous silica mass is not adequate to permit virtual elimination ofthe occluded bubbles, despite the fact that helium is characterized byone of the greatest rates of diffusion through not viscous silica as areknown. While hydrogen appears to have a slightly better diffusion ratethrough hot viscous silica, we have observed that it causes detrimentalefi'ects, e.g. volatilization of a reduced silicon oxide species andin-' creasing the water (or beta OI-I) content of the vitreous silicaproduct whereby its annealing point is lowered and devitrification isenhanced, both of which effects are undesirable for crucibles and otherproducts that are to be transparent and withstand the highest possibletemperatures without deformation. Accordingly, for our purposes,substituting hydrogen for helium is not satisfactory.

SUMMARY OF THE INVENTION We have now discovered that, by making certainunique and critical alterations in the old helium atmosphere firingprocess, the resultant transparent, vitreous silica bodies will bevirtually free of occluded bubbles. From our research, we observed thatthe green body of particulate silica material could be fired in heliumto an intermediate temperature below the melting point of cristobalitewhere some sintering of the body has occurred, but it still retainedabout 5-20 percent helium-filled porosity with pore diameters of about0.05 to 0.5 micron (as determined by conventional pressurized mercurypenetration porosimetry) and that communicates with the heliumatmosphere at the surface of the body. We further observed,surprisingly, that such partially sintered body could be further firedin argon atmosphere without the argon replacing the helium in the pores,but permitting the helium to substantially escape through the physicalcommunication paths of the interconnected (open) porosity to the ambientargon atmosphere as further sintering proceeded. The latter is evidencedby the resulting bodies being virtually free of occluded bubbles. Sinceit is known that argon has a lesser diffusion rate through hot viscoussilica, it is apparent that it did not enter the pores because it wouldhave been trapped there like the helium, only to a greater extent, tocause occluded bubbles in the final product. Thus, our uniquely improvedmethod involves a substantial change in ambient atmosphere from heliumto argon at a critical stage signifcantly before attaining completefusion of the silica mass (which is contrary to the noted prior art).

Our new method of making a transparent, vitreous silica body that is atleast virtually free of occluded bubbles includes the basic steps of:(a) forming a green body of substantially 60 mesh (advantageously -325mesh) particles of material selected from crystalline silica and glasshaving a SiO, content of at least about wt. percent, and (b) firing thegreen body up to and at temperature in the range of about 1,730 C. to2,200 C. to effect fusion and coalescence of the particles. However, thelatter step is now critically divided into two phases which involve: (a)conducting a first phase of the firing of the green body in an ambientatmo sphere of helium up to first temperature in the range of aboutl,600 C. to l,725 C. at a rate sufficient to effect sintering of thebody to a state in which the body has helium-filled pores that, aspredetermined by pressurized mercury penetration porosimetry:

i. communicate at the surface of the body with the ambient atmosphereii. occupy about 5 to 20 percent of the volume of the body and iii. havepore diameters in the range of about 0.05-0.5 micron, (b) thensubstantially changing the ambient atmosphere to one of argon, and (c)continuing the firing as a second phase thereof in the atmosphere ofargon from the first temperature up to second temperature in a range ofabout 1,730 C. to 2,200 C. (preferably about 1,800 C. to 1,900 C.) at arate sufficient to concurrently effect:

i. the escape of the helium from the pores substantially through thecommunication thereof with the argon atmosphere and ii. the virtualcollapse and elimination of those pores.

As is known, it is desirable to fire the green body relatively rapidlyto avoid or minimize formation of cristobalite, which would requireuneconomically extra firing at the higher temperatures to eliminate suchcrystalline phase for transparency. However, the rate of firing in ourfirst phase must be tempered to avoid temperature gradients through thebody that lead to fusion of the surface portions of the body which wouldclose the communication between the interconnected porosity in the bodyand the ambient atmosphere. An ordinary skilled worker in this art candetermine the rate appropriate for any desired size and shape of body tobe produced by modest and reasonable trials of varied rates in firstphase firings followed by subjecting the products of such first phasefirings to conventional pressurized mercury penetration porosimetry(e.g. see ASTM Bulletin No. 235, February 1959, pp. 39-44).

The rate of firing in our second phase must also be tempered to againavoid too quickly fusing the surface portions which would close off theinterconnected pores from the argon atmosphere before the helium inthose pores can substantially pass out therefrom via the physicalcommunication path of the pores with the argon temperature. Similarmodest and reasonable trials of second phase firing rates by a skilledworker will indicate an appropriate rate for any desired body size andshape, viz. a rate no faster than that by which the resultant product isvirtually free of occluded bubbles.

In the case of green bodies having thickness not greater than aboutone-fourth inch, we have found a very suitable procedure to be asfollows:

a. insert the green body into a hot zone having a helium atmosphere at afiring temperature in the range of about l,730 C. to 2,200 C.,

b. while maintaining the body in that hot zone and upon the bodyattaining intermediate temperature in the range of about l,600 C. tol,725 C., substantially change the atmosphere to one of argon, and

c. thereafter continue to maintain the body in that hot zone having theargon atmosphere until the body has attained the firing temperature.

For green bodies of thickness greater than about onefourth inch, we havefound the following procedure very satisfactory:

a. as a first phase of the firing, insert the green body into a hot zonehaving a helium atmosphere at first temperature in the range of aboutl,600 C. to l,725 C.,

b. maintain the body in that hot zone until the body has substantiallyuniformly attained the first temperature,

c. then substantially change the atmosphere to one of argon, and

d. as a second phase of the firing, increase the temperature of the hotzone, while maintaining the body therein, to second temperature in arange of about l,730 C. to 2,200 C. at a rate sufficient to concurrentlyeffect (i) the escape of helium from the pores as noted previously and(ii) the virtual collapse and elimination of those pores.

In the immediately preceding procedures, and for a given furnace andbody, the time at which the atmosphere change is to be made can bepredetermined by a trial firing of a body with suitable thermocouplearrangement for detecting when the body is at the intermediate or firsttemperature.

Suitable raw materials are any of the commonly available and heretoforeemployed substantially pure crystalline silica materials and/or highsilica glasses (including fused quartz). While substantially 60 meshparticles are a suitably practical maximum size for reasonable firingperiods to accomplish the desired properties, substantially 325 meshparticles (e.g. at least wt. percent of all particles being 325 mesh)are advantageous for attaining the most rapid suitable firing schedules.

The green bodies may be formed by any suitable or known ceramic formingprocess as desired. Examples of such processes that we deem particularlyuseful are slip casting, isostatic pressing and extrusion, of which slipcasting is our most preferred process for forming green crucibles.

According to conventional practice, the green body may be first calcinedat temperature of about 750l,000 C. to burn out organic impuritiespicked up from processing and handling of the raw materials. As desired,the previously noted firing procedure can be started either with the hotcalcined green body or with the calcined green body after it has beencooled to room temperature or some other temperature lower thancalcining temperature.

DESCRIPTION OF PREFERRED EMBODIMENTS In manufacturing crucibles andother articles of vitreous silica according to the new inventiondescribed herein, we usually employ the following known procedure forproducing the green articles:

1. Washing fused quartz cullet in HF-NHO acid solution,

2. Rinsing and drying of the washed cullet,

3. Dry-crushing washed cullet to about l4 mesh,

4. Wet-ball-milling (with distilled water) of --l 4 mesh crushed culletto yield fused quartz slip with particles of appropriate size (e.g. 95wt. percent of all particles being 325 mesh),

5. Casting slip into common plaster mold and allowoccluded bubbles wereproduced by employing the preceding green body forming procedure and themandrel support and gravity-stretching technique as described in ouraforementioned copending application, to which reference can be made forthe details thereof. The green and fired size characteristics of thesecrucibles are as follows:

Green Fired Outside diameter (in.): 6.555 5.940/6.000 Inside diameter(in.): 6295/6375 5.750 Wall thickness (in.): 0.090/0.l30 0.080/0.l00Length (in.): 6 5.74.0 Weight (grn.): 435125 435125 The calcined greencrucibles are ordinarily cooled to room temperature prior to startingthe firing procedure. A suitable furnace chamber or hot zone ispreheated to about 1,850 to 1,950 C. (e.g. approximately l,900 C.) andcontinuously flushed with helium. One of the calcined green crucibles isinserted into that preheated hot zone. As determined by a trial firingprocedure, the crucible will attain temperature in the range of l,600-l,725 C. approximately 100 1 10 seconds after being inserted into the hotzone. Accordingly, the usual procedure involves simultaneously turningoff the helium supply and turning on the argon supply when the cruciblehas been in the hot zone for 105 seconds, at which point the incomingargon will begin flushing out the helium in the hot zone andsubstantially replace it therein. The crucible is further maintained inthat hot zone for another 105 seconds, after which it is moved to amuffle chamber to cool. The resultant crucible is virtually free ofoccluded bubbles (i.e., those visible to the human eye).

When another crucible is made in the same way as the above-notedcrucible, except that helium is continued in the hot zone until afterthe crucible has attained temperature of l,730 C. or higher (eitherfollowed or not followed by a change over to argon in the hot zone),that other crucible (after cooling) will commonly contain a substantialnumber of readily visible occluded bubbles randomly scattered within it.

EXAMPLE 2 Transparent, vitreous silica solid cylinders virtually free ofoccluded bubbles were also produced by employing the same green bodyforming procedure as in the previous example. The slip-cast greencylinders had a diameter of 1.1 inches and a height of 1.1 inches.

A furnace hot zone was preheated to about l,750-l ,760" C. andcontinuously flushed with helium. Some of the green cylinders wereinserted into the hot zone with a graphite radiation shield between themand the graphite susceptor for more even heating of the cylinders. Upontheir insertion, the hot zone temperature dropped into the range ofl,600-l,725 C. After minutes, the cylinders had attained temperature inthe latter range throughout their cross-section and the atmosphere gaswas changed to continuously flushing argon instead of the helium. Thenthe hot zone temperature was gradually increased to l,800 C. over aperiod of about one hour (at the end of which the cylinders had attainedl,800 C.), and finally increased to 1,920" C. within another 5 minutesto assure elimination of any possible presence of cristobalite. Aftercooling, the resultant vitreous silica cylinders were thoroughlytransparent and free of occluded bubbles.

Another two of the green cylinders were fired during the first or heliumphase thereof in the same manner as were the above-noted cylinders.However, upon the change-over to argon in the hot zone, the furnacetemperature was rapidly increased to 1,920 C. within about 4 minutes andheld there for times of about 40 minutes for one of the cylinders and ofabout 155 minutes for the other cylinder. After cooling of each, thesecylinders exhibited a massive group of occluded bubbles within theircentral portions. Thus, the rate of increase to l,920 C. was too rapidand caused fusion of the surface portions of the cylinders wherebyhelium was trapped in the closed porosity and not even significantlyreduced after 155 minutes at high temperature in argon.

Another one of the green cylinders was inserted into the hot zonepreheated to about 1,770 C. and continuously flushed with helium. Afteran initial drop in hot zone temperature to somewhat below l,760 C., thattemperature recovered to l,760 C. during a period of 10 minutes from thetime of inserting the green cylinder therein. At least the surfaceportion of this cylinder had attained about 1,760 C. at that time, atwhich point the helium was turned off and argon was then continuouslyflushed through the hot zone. Following this atmosphere change-over, thehot zone temperature was gradually increased to l,800 C. over a periodof 25 minutes. Thereafter, this cylinder was cooled and found to exhibita large number of occluded bubbles within its central portion. Thisresult demonstrates the fact that merely heating a silica body up tofusion temperature thereof in helium (thereby closing off the porestherein filled with helium) and thereafter surrounding it with argon fora substantial period of time does not yield the desired result ofvirtual freedom from occluded bubbles in the resultant vitreous silicabody. Of course, without the change-over to argon from helium,-at fusiontemperatures, the results are even worse, viz. the body is very opaquedue to massive numbers of occluded bubbles.

Mesh sizes stated herein are according to the Tyler sieve scale.

We claim:

1. The method of making a transparent, vitreous silica body that is atleast virtually free of occluded bubbles,

l. which includes the basic steps of a. forming a green body ofsubstantially 60 mesh particles of material selected from crystallinesilica and glass having a SiO content of at least about wt. percent, and

b. firing said green body up to and at temperature in the range of aboutl,730 to 2,200 C. to effect fusion and coalescence of said particles,and

2. wherein the improvement comprises a. conducting a first phase of saidfiring of said green body in an ambient atmosphere of helium up to firsttemperature in the range of about 1,600" to l,725 C. at a ratesufficient to effect sintering of said body to a state in which saidbody has helium-filled pores that, as predetermined by pressurizedmercury pentration porosimetry,

i. have open communication at the surface of said body with saidatmosphere,

ii. occupy about 5 to 20 percent of the volume of said body and iii.have pore diameters in the range of about 0.05 to 0.5 micron,

b. then substantially changing said atmosphere to one of argon, and

c. continuing said firing as a second phase thereof in said atmosphereof argon from said first temperature up to second temperature in a rangeof about l,730 to 2,200 C. at a rate sufficient to concurrently effecti. the escape of the helium from said pores substantially through saidcommunication with said atmosphere and ii. the virtual collapse andelimination of said pores.

2. The method of claim 1 wherein said second temperature is in the rangeof about l,800 to 1,900 C.

3. The method of claim 1 wherein said particles are substantially -325mesh.

4. The method of claim 1 wherein said green body is formed by slipcasting said particles.

5. The method of making a thin, transparent, vitreous silica body thatis at least virtually free of occluded bubbles,

l. which includes the basic steps of a. forming a green body ofthickness not greater than about inch and of substantially 60 mesh ofmaterial selected from crystalline silica and glass having a Si contentof at least about 90 wt. percent, and

b. firing said green body up to and at temperature in the range of about1,730 to 2,200 C. to effect fusion and coalescence of said particles,and

2. wherein the improvement comprises a. inserting said green body into ahot zone having a helium atmosphere at a firing temperature in the rangeof about 1,730 to 2,200 C.,

b. while maintaining said body in said hot zone and upon said bodyattaining intermediate temperature in the range of about l,600 to l,725C.,

substantially changing said atmosphere to one of argon, and

c. thereafter continuing to maintain said body in said hot zone havingsaid argon atmosphere until said body has attained said firingtemperature.

6. The method of claim 5 wherein said firing temperature is in the rangeof about l,800 to 1,900 C.

7. The method of claim 5 wherein said particles are substantially 325mesh.

8. The method of making a transparent, vitreous silica body that is atleast virtually free of occluded bubbles,

l. which includes the basic steps of a. forming a green body ofthickness greater than about /4 inch and of substantially 60 meshparticles of material selected from crystalline silica and glass havinga SiO content of at least about weight percent, and b. firing said greenbody up to and at temperature in the range of about l,730 to 2,200 C. toeffect fusion and coalescence of said particles, and 2. wherein theimprovement comprises a. as a first phase of said firing, inserting saidgreen body into a hot zone having a helium atmosphere at firsttemperature in the range of about l,600 to 1,725 C., b. maintaining saidbody in said hot zone until said body has substantially uniformlyattained said first temperature, 0. then substantially changing saidatmosphere to one of argon, and d. as a second phase of said firing,increasing the temperature of said hot zone, while maintaining said bodytherein, to second temperature in a range of about 1,730 to 2,200 C. ata rate sufficient to concurrently effect i. the escape of the heliumfrom pores in said body substantially through open communication of saidpores with said atmosphere at the surface of said body and ii. thevirtual collapse and elimination of said pores.

9. The method of claim 8 wherein said second temperature is in the rangeof about l,800 to 1,900 C.

10. The method of claim 8 wherein said particles are substantially 325mesh.

2. wherein the improvement comprises a. conducting a first phase of saidfiring of said green body in an ambient atmosphere of helium up to firsttemperature in the range of about 1,600* to 1,725* C. at a ratesufficient to effect sintering of said body to a state in which saidbody has helium-filled pores that, as predetermined by pressurizedmercury pentration porosimetry, i. have open communication at thesurface of said body with said atmosphere, ii. occupy about 5 to 20percent of the volume of said body and iii. have pore diameters in therange of about 0.05 to 0.5 micron, b. then substantially changing saidatmosphere to one of argon, and c. continuing said firing as a secondphase thereof in said atmosphere of argon from said first temperature upto second temperature in a range of about 1,730* to 2,200* C. at a ratesufficient to concurrently effect i. the escape of the helium from saidpores substantially through said communication with said atmosphere andii. the virtual collapse and elimination of said pores.
 2. The method ofclaim 1 wherein said second temperature is in the range of about 1,800*to 1,900* C.
 2. wherein the improvement comprises a. inserting saidgreen body into a hot zone having a helium atmosphere at a firingtemperature in the range of about 1, 730* to 2,200* C., b. whilemaintaining said body in said hot zone and upon said body attainingintermediate temperature in the range of about 1,600* to 1,725* C.,substantially changing said atmosphere to one of argon, and c.thereafter continuing to maintain said body in said hot zone having saidargon atmosphere until said body has attained said firing Temperature.2. wherein the improvement comprises a. as a first phase of said firing,inserting said green body into a hot zone having a helium atmosphere atfirst temperature in the range of about 1,600* to 1,725* C., b.maintaining said body in said hot zone until said body has substantiallyuniformly attained said first temperature, c. then substantiallychanging said atmosphere to one of argon, and d. as a second phase ofsaid firing, increasing the temperature of said hot zone, whilemaintaining said body therein, to second temperature in a range of about1,730* to 2,200* C. at a rate sufficient to concurrently effect i. theescape of the helium from pores in said body substantially through opencommunication of said pores with said atmosphere at the surface of saidbody and ii. the virtual collapse and elimination of said pores.
 3. Themethod of claim 1 wherein said particles are substantially -325 mesh. 4.The method of claim 1 wherein said green body is formed by slip castingsaid particles.
 5. The method of making a thin, transparent, vitreoussilica body that is at least virtually free of occluded bubbles,
 6. Themethod of claim 5 wherein said firing temperature is in the range ofabout 1,800* to 1,900* C.
 7. The method of claim 5 wherein saidparticles are substantially -325 mesh.
 8. The method of making atransparent, vitreous silica body that is at least virtually free ofoccluded bubbles,
 9. The method of claim 8 wherein said secondtemperature is in the range of about 1,800* to 1,900* C.
 10. The methodof claim 8 wherein said particles are substantially -325 mesh.